This invention relates to an oxygen ion conductive solid electrolyte, which is a partially stabilized zirconia essentially consisting of ZrO.sub.2 and Y.sub.2 O.sub.3 that is suitable for use in oxygen sensors or oxygen sensitive elements of air/fuel ratio detectors to be disposed in exhaust systems of combustion engines, and a method of producing same.
In using ZrO.sub.2 as an oxygen ion conductive solid electrolyte it is usual to stabilize or partially stabilize zirconia with addition of a suitable oxide which forms a solid solution with zirconia, and Y.sub.2 O.sub.3 has widely been used as the stabilizing oxide. Stabilized zirconia, which consists of cubic crystals, is relatively low in thermal shock resistance and hence is liable to crack when subjected to rapid heating. For improvement in this regard, partially stabilized zirconia consisting of cubic, tetragonal and monoclinic crystals has been developed.
However, in a specific region of temperature (200.degree.-300.degree. C.) partially stabilized zirconia undergoes aging deterioration and consequently lowers in mechanical strength. The fundamental cause of this problem is the transformation of tetragonal crystal grains in partially stabilized zirconia into monoclinic crystal grains. The transformation is accompanied by some expansion of the volume of the ceramic body, and the volumetric expansion causes micro-cracks.
To solve the problem of aging deterioration of partially stabilized zirconia, JP-A 56-134564 proposes producing a partially stabilized zirconia which contains Y.sub.2 O.sub.3 that is mainly constituted of tetragonal crystals and cubic crystals (but may include some monoclinic crystals) by taking measures to make the crystal grains of the sintered product smaller than 2 .mu.m in mean grain size to thereby prevent transformation of the tetragonal crystals to monoclinic crystals. It is intended to reduce the crystal grain size in the sintered product by accomplishing sintering at a relatively low temperature, considering that sintering at temperatures above 1500.degree. C. causes unwanted growth of crystal grains and hence gives a sintered body which contains relatively large crystal grains and is relatively low in mechanical strength and thermal shock resistance. According to the need a sintering aid such as silica or alumina is used.
JP 60-5548 shows using silica and alumina jointly as sintering aids in producing a solid electrolyte of zirconia partially stabilized by yttria.
However, we have recognized that solid electrolytes of partially stabilized zirconia hitherto developed are too low in withstand voltage. When the partially stabilized zirconia is used as a solid electrolyte of, for example, an oxygen ion pump element of an oxygen sensor a so-called blackening phenomenon occurs even at a voltage lower than 3 V whereby the oxygen sensor can no longer perform accurate detection of oxygen concentrations. In our view the primary reason for the insufficiency of withstand voltage is the presence of a considerable amount of SiO.sub.2 in the ZrO.sub.2 -Y.sub.2 0.sub.3 solid electrolyte.